Remember that there are two basic versions of the 9550, the one with a 128-bit memory pathway, and the one with a 64-bit memory pathway. That's the number of data lanes, not the size, you could get a 256MB card with a 64-bit pathway, or a 128MB card with a 128-bit pathway.
The 128-bit version is just an underclocked 9600. Overclock ability will depend on the RAM, 5ns is only rated for 200MHz, 4ns is rated for 250MHz. The former poster likely has 4ns RAM, which was rated for 250MHz, overclocked to 260MHz, which is DDR520.
You could get two cards with the same memory clock of 200MHz (DDR400) but one might be using chips with 250MHz rating, while the other isn't.
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